1. Field of the Invention
This invention relates to driving devices for moving the photographic lens barrel of a camera stepwise.
2. Description of the Prior Art
Devices of this kind generally operate in such a manner that prior to making an exposure, the lens barrel is axially moved forward to focus on an object to be photographed, and after the exposure is completed, it is further moved forward until one cam moves away from the follower on the lens barrel so that the lens barrel is set in the initial position of the next cam and at the same time the drive circuit is rendered inoperative.
To detect when the lens barrel reaches the initial position of the next cam, it has been the common practice in the prior art that while the number of ratchet teeth in each cam is being memorized, after the exposure is completed, a corresponding number of cycles of reciprocating operation to that obtained by subtracting the required number of cycles of reciprocating operation for the focusing adjustment from the total number of ratchet teeth in each cam, is counted as the lens barrel further moves axially forward. When the counting is completed, the drive circuit stops the supply of driving power to the ratchet mechanism. In this case, however, if it happens that one of the total sum of the cycles of reciprocating operation failed to advance the distance adjusting ring by the distance of one ratchet tooth, for example, through no teeth, or two teeth at a time, the phase of rotation of the distance adjusting ring errs with the result that for all subsequent exposures, the focusing of the photographic lens will not accurately be adjusted.